The scene where Rosie and Alex are lying on Rosie's bed with baby Katie was not originally planned. The baby actress was asleep on the set and Lily Collins and Sam Claflin were on the bed with her, waiting while she rested. Director Christian Ditter decided to improvise and put it into the movie.
Director Christian Ditter allowed Lily Collins and Sam Claflin to improvise the scene where Rosie and Alex walk the streets of Boston after their night out. Both actors said that this was their favorite moment of the shoot.
Cecelia Ahern had a cameo but was cut out from the film. This was due to the fact that director, Christian Ditter noticed people whispering about whether or not it was Ahern in the background during the film's screening. He felt that her cameo was a distraction to Rosie's dialogue and the importance of the scene.
The decision to set the film in England, even though the novel it's based on is set in Ireland, and filming of the England scenes took place in Ireland (in and around Dublin), was made because so many of the actors were English.
Sam Claflin and Lily Collins have both played major parts in widely popular books that were adapted to movies. Claflin played Finnick Odair in The Hunger Games: Catching Fire, The Hunger Games: Mockingjay - Part 1 and The Hunger Games: Mockingjay - Part 2 as well as Will Traynor in Me Before You and Collins played Clary Fray in The Mortal Instruments: City of Bones.
The trivia items below may give away important plot points.
Sam Claflin's scenes were filmed during the first weeks of production, and when he left halfway through the shoot to make The Riot Club (2014), Lily Collins remained and filmed the scenes of Rosie being pregnant and at home in England without Alex. She said that when Sam Claflin left, she really felt like Alex had left Rosie.